The stimulus here seemed vague. It says that brown dwarfs are identified on the basis of whether they have lithium in their atmosphere. Then, it goes on to tell us that the brown dwarf cannot consume its lithium due to the lack of a functional internal furnace. I see why C is correct if we've already determined that all brown dwarfs have lithium in their atmosphere. Yet, since that has not been established, it seems like a big jump to assume that because the internal furnace cannot consume lithium there must be lithium present. Indeed, if they did not have lithium in the atmosphere at any point, the inability to process it would be irrelevant and the brown dwarf could be without it, which would render C wrong.

Thanks so much for sharing that advice!